I am looking for an efficient bookshelf speakers for my Stello S100 (50W) power amp! I will not use a sub woofer, so I need a very good lower bass response. Most bookshelves do not have frequency response below 40-50Hz. I need it to respond to deepest bass notes with flat response!
Any suggestions to buy in UK?

My bookshelves extend down to 35Hz, and I don't think there's much to gain from going further down. What matters is the quality of the bass, which you can get from sealed enclosures. Ported or bass reflex enclosures have deeper but looser less authoritative bass, which is no thanx. With 50W amp, efficiency should not be a factor at all. I've heard 8W SET amps get really loud on medium efficiency speakers. Those efficient speakers tend to have less than 1mm of xmax, so the bass output is very limited. The woofer should have at least 6mm of xmax if you want extension to below 40Hz and tight, authoritative bass. I recommend a sealed enclosure speaker with a 7" high output woofer/midrange. Expect efficiency to be about 87-89dB, but as usual manufacterers inflate their ratings by around 3dB.

I am looking for an efficient bookshelf speakers for my Stello S100 (50W) power amp! I will not use a sub woofer, so I need a very good lower bass response. Most bookshelves do not have frequency response below 40-50Hz. I need it to respond to deepest bass notes with flat response!
Any suggestions to buy in UK?

You're asking for something that for the most part doesn't exist. If you need "very good" bass response below 40-50 Hz, you won't find it in a bookshelf speaker. You could try a design using the Adire Audio Extremis, like the DIYCable Kit 61, but don't delay too long, since Adire Audio went out of business and the supply of Extremis drivers is scarce.

kaushama you can do much better than the Pioneers. As mentioned above, the Adire Audio Extremis can do it, but I don't like the midrange which is muddy. I use the Dayton RS180 which I measured to extend ruler flat to 40Hz and then drop off slowly in response. A good commercial bookshelf would be the Magico Mini's.

But 35Hz at what response? Usually it would be at -10dB, which means it doesn't really extend to 35Hz. You need to look at the actual frequency response graph. Here is what I measured for my speakers, (btw the two dips in the bass are not really there but due to the measurement technique):

As you can see, modern 2-way bookshelves have very good bass extension.

That doesn't mean anything. Most earbuds have specs that claim response down to 12Hz. Usually when you see that kind of low response for bookshelf speakers, it means -10dB (sometimes worse) and often includes some amount of room gain. There are limits to what a bookshelf speaker can do.